1. Field of the Invention
This invention relates to a radiocommunication device used in a digital mobile communication system such as a mobile telephone system, cordless telephone system or radio local area network (LAN) system, for example, and more particularly to a device utilizing a spread spectrum communication system.
2. Description of the Related Art
In recent years, a spread spectrum communication system which is hardly affected by interference or disturbance has received much attention as one communication system applied to the mobile communication system.
The spread spectrum communication system is used to realize a CDMA (Code Division Multiple Access) system. In the transmission side device, the CDMA system modulates digitized audio data or image data based on the digital modulation scheme such as the PSK modulation scheme, converts the modulated transmission data into a wide-band base band signal by use of a spread code such as a pseudorandom noise code (PN code), then up-converts the signal to a radio frequency signal and transmits the radio frequency signal. In the reception side device, the received radio frequency signal is down-converted to a signal of intermediate frequency or base band frequency, then the signal is subjected to the inverse spreading process by use of the same code as the spread code used in the transmission side device, the signal is subjected to the digital demodulation based on the digital demodulation scheme such as the PSK demodulation scheme, and thus the reception data is reproduced.
In the above type of system, a RAKE reception scheme is used as one of the measures for coping with multipath. That is, in the mobile communication system, the radio carrier wave transmitted from the transmission side device may directly reach the reception side device or reach the reception side device after being reflected from buildings or mountains. If one radio carrier wave reaches the reception side device via a plurality of paths, the radio carrier waves transmitted via the plurality of paths are vectorially synthesized at the antenna end portion of the reception side device, thereby lowering the reception level. This phenomenon is called the multipath. If the multipath occurs, the radio signal cannot be received at all by the reception device in some cases in a system in which the bandwidth of the radio channel is approximately 30 kHz and narrow, for example. However, in the spread spectrum communication system, since the bandwidth of the radio channel is wide, the radio signal can always be received although it is partly lost.
Therefore, in the spread spectrum radiocommunication system, a RAKE reception scheme is used. The RAKE reception scheme has a plurality of independent demodulators and separates a multipath reception signal received by one antenna for each spread code length (one chip) unit and supplies the same to the demodulators. Then, the reception signal is demodulated by effecting the inverse spreading process by use of spread codes for the respective paths in the respective demodulators and reception data is reproduced by synthesizing the symbols of the demodulated reception signals in the plurality of paths. The path diversity can be effected by using the RAKE reception scheme. Therefore, the reception quality can be significantly enhanced when the multipath occurs.
However, in the conventional RAKE reception scheme, when spread codes of chip phases synchronized with the phases of the respective paths are allotted to the respective demodulators, spread codes generated from a common spread code generator are delayed by necessary chip numbers by use of a delay circuit and then allotted, for example. That is, the chip phase of the spread code is set to an optimum value by changing the delay amount of the delay circuit. Therefore, it is difficult to precisely set the chip phase. Further, the delay circuit is generally constructed by shift registers. Therefore, the maximum value of the delay amount is determined by the number of stages of the shift registers and the circuit scale is increased if the maximum value of the delay amount is further increased.